RNA interference (RNAi) refers to the cellular process of sequence specific, post-transcriptional gene silencing in animals mediated by small inhibitory nucleic acid molecules, such as a double-stranded RNA (dsRNA) that is homologous to a portion of a targeted messenger RNA (Fire et al., Nature 391:806, 1998; Hamilton et al., Science 286:950-951, 1999). RNAi has been observed in a variety of organisms, including mammalians (Fire et al., Nature 391:806, 1998; Bahramian and Zarbl, Mol. Cell. Biol. 19:274-283, 1999; Wianny and Goetz, Nature Cell Biol. 2:70, 1999). RNAi can be induced by introducing an exogenous synthetic 21-nucleotide RNA duplex into cultured mammalian cells (Elbashir et al., Nature 411:494, 2001a).
The mechanism by which dsRNA mediates targeted gene-silencing can be described as involving two steps. The first step involves degradation of long dsRNAs by a ribonuclease III-like enzyme, referred to as Dicer, into short interfering RNAs (siRNAs) having from 21 to 23 nucleotides with double-stranded regions of about 19 base pairs and a two nucleotide, generally, overhang at each 3′-end (Berstein et al., Nature 409:363, 2001; Elbashir et al., Genes Dev. 15:188, 2001b; and Kim et al., Nature Biotech. 23:222, 2005). The second step of RNAi gene-silencing involves activation of a multi-component nuclease having one strand (guide or antisense strand) from the siRNA and an Argonaute protein to form an RNA-induced silencing complex (“RISC”) (Elbashir et al., Genes Dev. 15:188, 2001). Argonaute initially associates with a double-stranded siRNA and then endonucleolytically cleaves the non-incorporated strand (passenger or sense strand) to facilitate its release due to resulting thermodynamic instability of the cleaved duplex (Leuschner et al., EMBO 7:314, 2006). The guide strand in the activated RISC binds to a complementary target mRNA, which is then cleaved by the RISC to promote gene silencing. Cleavage of the target RNA occurs in the middle of the target region that is complementary to the guide strand (Elbashir et al., 2001b).
What is needed are alternative effective therapeutic modalities useful for treating or preventing diseases or disorders by regulating the expression of genes and other nucleic acid based regulatory systems in a cell.
A need therefore exists for nucleic acid compounds having enhanced stability that are useful in various therapeutic modalities such as RNA interference.